1. Field of the Invention
The present invention relates to telecommunications systems and, particularly, to an improved fault tolerant H.323 system.
2. Description of the Related Art
The International Telecommunications Union (ITU) Recommendation H.323 describes a set of devices and protocols for multimedia communication over packet-switched networks. The four main components defined by the specification are clients (also referred to as terminals or endpoints), multipoint control units, gateways and gatekeepers.
The H.323 Recommendation provides for separate H.225.0 call signaling, H.245 call control and media channels. Typically, H.225.0 call signaling and H.245 call control are routed through the gatekeeper, while the media channels (i.e., audio, data and/or video) are routed directly between the endpoints. For example, to place a call between two clients, the calling party client sends a message to a gatekeeper, which resolves the address of the receiving party and sends the appropriate signaling messages to the caller and receiver. Once the signaling and control channels have been established, the endpoints establish the media channels. There may be more than one media channel for a call, such as two unidirectional channels for an audio call. The H.225.0 signaling channel and the H.245 call control channel are also used to terminate the call.
If the gatekeeper fails for any reason, the endpoints become aware of the situation because their connection to the gatekeeper (usually seen in the client as a TCP socket) will close. When the H.245 control channel is detected as having been closed, the H.323 Recommendation requires the endpoints to disconnect the call and close the media channel(s). This requirement exists so that when the gatekeeper does recover, there are no outstanding media connections still alive of which the gatekeeper is unaware. Thus, the network is prevented from perpetually maintaining a call. When a bandwidth policy is employed, the network is prevented from allowing more calls than the network can handle.
While redundant back-up gatekeepers which track other gatekeepers"" operations have been known to be provided, such systems do not allow for preservation of ongoing calls. In these cases, ongoing calls are lost and the H.323 client terminals must re-register and re-initiate call setup again, which can cause a burst of excessive load on the network and on the gatekeeper. Moreover, gatekeeper to gatekeeper communication can consume processing power and load the system.
These disadvantages in the prior art are overcome in large part by a system and method according to the present invention. In particular, H.323 client terminals according to the present invention include redundancy supervisory layers which provide redundant H.225.0 call signaling and H.245 call control to secondary gatekeepers. The secondary gatekeepers respond as if they were the primary gatekeeper, but the H.323 client terminals do not set up media channels as part of the call setup via the secondary gatekeepers. If the primary gatekeeper fails, the media channel(s) is maintained, and the redundancy supervisory layers know that the call signaling and call control channels are to be handled through the secondary gatekeeper.
An H.323 client terminal according to an embodiment of the invention employs primary and secondary H.323 control units or state machines. The H.323 client terminal registers with both a primary and a secondary gatekeeper. The primary control unit sends signaling messages to a primary gatekeeper and triggers the secondary control unit to send messages with the appropriate call identifier to a secondary gatekeeper. The primary control unit establishes a call per standard H.323 protocols. As the secondary control unit receives signaling from the secondary gatekeeper, the secondary control unit checks the status of the call with the primary control unit. If the call signaling on the primary control unit is active, no further action is taken by the secondary control unit once the secondary call is established. If the call signaling with the primary gatekeeper fails, the secondary control unit takes over communication.